To drive on a busy freeway in close proximity to other cars, ignoring the multiple open lanes. Usually entails drivers clustering to the right side of the freeway while two or more left hand lanes are open, causing a dangerous and just stupid scenario at high speeds, greatly increasing the probability of an accident, which is usually caused by one person hitting their brakes, getting rear-ended, losing control of their car. This process repeats for the car that caused the rear-ending, as he/she is then rear-ended, and this goes into a domino effect of destruction, chaos, and sometimes death. Which in some circles, is recognized as Traffic Darwinism, as the less intelligent drivers are sometimes killed by their own stupidity.

Other definitions:

A dense orgy of more than three people where positioning movement is difficult. Often consists of double penetrations and more skin touching than air.

WTF is up with these people! Get out of the way! Look at all that freeway over there! Bunch of cluster fuckers!

I almost got hit by 3 cars on the freeway because all these idiots kept on cluster fucking the road!

Sally really knows how to cluster fuck! I didn't know there were that many places to put a penis!